The use of portable electronic instruments is increasing as electronic equipment gets smaller and lighter due to developments in the high-tech electronic industry. Studies of lithium secondary batteries are actively being pursued in accordance with the increased need for a battery having high energy density for use as a power source in these portable electronic instruments. Such a lithium secondary battery, having an average discharge potential of 3.7 V (i.e., a battery having substantially a 4 V average discharge potential), is considered to be an essential element in the digital generation since it is an indispensable energy source for portable digital devices such as cellular telephones, notebook computers, camcorders, also known as the “3C” devices.
A lithium secondary battery produces electric energy from a change of chemical potential of active materials during intercalation/deintercalation reactions of lithium ions at negative and positive electrodes.
Lithium secondary batteries use materials that reversibly intercalate or deintercalate lithium ions during charge and discharge reactions as both positive and negative active materials, and they include a liquid or polymer electrolyte between the negative and positive electrodes. Exemplary positive active materials include lithium metal oxide, and exemplary negative active materials include lithium metals, lithium-containing alloys, crystalline or amorphous carbons, and carbon-containing composites.
The choice of suitable electrolytes is one of the factors for improving cell characteristics, because reactions between electrodes and the electrolyte have an effect on cell performance. To improve low temperature characteristics, a lithium secondary battery using a liquid electrolyte uses an organic solvent with a low boiling point that induces swelling of a prismatic or pouch battery during high temperature storage. As a result, the reliability and safety of the battery deteriorate at a high temperature.
For solving the internal pressure problem, there is disclosed a method in which the safety of a secondary battery including a non-aqueous electrolyte is improved by mounting a vent or a current breaker for ejecting internal electrolyte solution when the internal pressure is increased above a certain level. However, a problem with this method is that mis-operation may result from an increase in internal pressure itself.
Furthermore, a method in which the SEI-forming reaction is changed by injecting additives into an electrolyte so as to inhibit the increase in internal pressure is known. For example, Japanese Patent Laid-open No. 97-73918 discloses a method in which high temperature storage characteristics of a battery are improved by adding 1% or less of a diphenyl picrylhydrazyl compound to the electrolyte. Japanese Patent Laid-open No. 96-321312 discloses a method in which cycle life and long-term storage characteristics are improved using 1 to 20% of an N-butyl amine based compound in an electrolyte. Japanese Patent Laid-open No. 96-64238 discloses a method in which storage characteristics of a battery are improved by adding 3×10−4 to 3×10−3 M of calcium salt to the electrolyte. Japanese Patent Laid-open No. 94-333596 discloses a method in which storage characteristics of a battery are improved by adding an azo-based compound to inhibit the reaction between the electrolyte and the negative electrode of the battery. In addition, Japanese Patent Laid-open No. 95-176323 discloses a method in which CO2 is added to an electrolyte, and Japanese Patent Laid-open No. 95-320779 discloses a method in which sulfide-based compounds are added to an electrolyte in order to prevent the electrolyte from decomposing.
Such methods as described above for inducing the formation of an appropriate film on a negative electrode surface such as an organic SEI film by adding a small amount of organic or inorganic materials in order to improve the storage characteristics and safety of a battery. However, there are various problems with these methods. For example, the added compound decomposes or forms an unstable film by interacting with the carbon negative electrode during the initial charge and discharge due to inherent electrochemical characteristics, resulting in the deterioration of the ion mobility in electrons. Also, gas is generated inside the battery such that there is an increase in internal pressure, resulting in significant deterioration of the storage, safety, cycle life, and capacity characteristics of the battery.
In order to inhibit swelling that is induced from use of liquid electrolyte, it is suggested to use a polymer solid electrolyte. The polymer solid electrolyte leaks less than liquid electrolyte, resulting in improvement of battery safety.
However, the polymer solid electrolyte has a lower ionic conductivity than a liquid electrolyte. A linear polymer or cross-linked polymer of a homopolymer or copolymer having ethylene oxide as a base unit has been used as a monomer of an ionic conductive polymer for forming a polymer solid electrolyte. The polymer derived from such a monomer is likely to crystallize, however, and thus has poor properties at low temperatures. Therefore, the polymer solid electrolyte has a limit for inhibition of battery swelling.